High-strength martensite heat resisting cast steel, method of producing the steel, and applications of the steel

ABSTRACT

A high-strength martensite heat resisting steel which has long-time creep rupture strength required for steam temperature condition of 600-630° C. and toughness at room temperature, and which is suitable for use as a material of a steam turbine rotor shaft and as large-sized forged steel with an improvement of hot forgeability. A method of producing the steel and applications of the steel are also provided. The high-strength martensite heat resisting steel contains 0.05-0.20% by mass of C, 0.1% or less of Si, 0.05-0.6% of Mn, 0.1-0.6% of Ni, 9.0-12.0% of Cr, 0.20-0.65% of Mo, 2.0-3.0% of W, 0.1-0.3% of V, 2.0% or less of Co, 0.02-0.20% of Nb, 0.015% or less of B, 0.01-0.10% of N, and 0.015% or less of Al, (W/Mo) being 4.0-10.0.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a novel high-strength martensite heatresisting steel which has superior creep rupture strength at hightemperatures of 600-630° C. and which is suitable for use as large-sizedforged steel, and to a method of producing the novel steel. Also, thepresent invention relates to a rotor shaft of a steam turbine, a methodof producing the rotor shaft, a rotor blade and a stator nozzle of thesteam turbine, and a steam turbine power plant.

2. Description of the Related Art

Materials having superior high-temperature strength are required forvarious members of a steam turbine which are exposed to hightemperatures. In reply to such requirement, a practically used materialof a steam turbine rotor shaft has been changed from CrMoV steel to 12Crsteel, i.e., ferrite-base heat resisting steel having superiorhigh-temperature strength. The steam turbine rotor shaft is required tohave not only long-time creep rupture strength, but also toughness atroom temperature to bear against stress abruptly applied when the steamturbine is started. If the toughness at room temperature is low, thereis a risk that the rotor shaft may cause brittle rupture at the start ofthe steam turbine. For that reason, improvements of heat resisting steelhave been progressed in recent years. In particular, ferrite-base 12Crheat resisting steels having been developed for use at steam temperatureof 600° C. or above are disclosed in Patent Document 1 (JP,A 62-103345),Patent Document 2 (JP,A 2-290950), Patent Document 3 (JP,A 4-147948),Patent Document 4 (JP,A 7-34202), and Patent Document 5 (JP,A2000-54803).

Meanwhile, steam turbines have recently been improved with intent torealize higher efficiency and larger capacity, but a thermal power plantoperating at steam temperature of 650° C. is not yet realized. Thereasons reside in not only the state of the art that thehigh-temperature material technology for the entire plant is stillinsufficient, but also the problem of reduction in the material costwhich is necessitated from a market trend toward a lower cost. Theabove-described materials of the turbine rotor shaft adapted for steamtemperature of 600° C. or above are relatively high in cost. A primaryone of factors pushing up the cost is poor productivity. Because thematerials of the turbine rotor shaft are susceptible to strengthening ofthe grain boundary and enlargement and aggregation of M₂₃C₆ carbides, Bis added to those materials to prevent aggregation into coarser grains.In producing a large-sized forged product, however, B noticeably reducesproductivity because of increasing forging resistance and narrowing aforgeable temperature range. Thus, the production cost is increased.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a high-strengthmartensite heat resisting steel which has long-time creep rupturestrength required for steam temperature condition of 600-630° C. andtoughness at room temperature, and which is suitable for use as amaterial of a steam turbine rotor shaft and as large-sized forged steelwith an improvement of hot forgeability, and to a method of producingthat steel. Another object of the present invention is to provide asteam turbine rotor shaft and a method of producing it, a steam turbinerotor blade and a method of producing it, a steam turbine stator nozzleand a method of producing it, as well as a steam turbine and a steamturbine power plant, including the method of producing the steamturbine, in which the turbine blade in a stage using steam to cool therotor has a larger height by increasing the high-temperature tensilestrength, and higher thermal efficiency is ensured.

First, by selecting 620° C. as the target temperature in use of heatresisting steel to be developed, the inventors studied influences of Ni,Mo, W and B upon creep rupture strength at 620° C. and toughness at roomtemperature. As a result, the inventors found respective compositionranges of added elements, which satisfied the required creep rupturestrength at 620° C. and 10⁵ hours and had superior toughness at roomtemperature, thereby accomplishing the steel according to one aspect ofthe present invention. Further, the inventors produced various kinds ofsteels while changing the Co content with respect to those compositionranges, and studied influences upon the creep rupture strength at 620°C., the toughness, and the tensile strength, thereby accomplishing thesteel according to another aspect of the present invention.

According to one aspect, the present invention resides in ahigh-strength martensite heat resisting steel containing 0.05-0.20% bymass of C, 0.1% or less of Si, 0.15-0.7% of Mn, 0.15-1.0% of Ni,9.5-12.0% of Cr, 0.20-0.65% of Mo, 2.0-3.0% of W, 0.1-0.3% of V,0.03-0.15% of Nb, and 0.01-0.10% of N, (W/Mo) being 4.0-10.0, thebalance being Fe and unavoidable impurities, and also resides in a steamturbine rotor shaft, a rotor blade and a stator nozzle each using thatsteel.

According to another aspect, the present invention resides in ahigh-strength martensite heat resisting steel containing 0.05-0.20% bymass of C, 0.1% or less of Si, 0.15-0.7% of Mn, 0.15-1.0% of Ni,9.5-12.0% of Cr, 0.20-0.65% of Mo, 0.1-2.0% of Co, 1.8-3.0% of W,0.1-0.3% of V, 0.03-0.15% of Nb, and 0.01-0.10% of N, (W/Mo) being4.0-10.0, the balance being Fe and unavoidable impurities, and alsoresides in a steam turbine rotor shaft, a rotor blade and a statornozzle each using that steel.

Preferably, the high-strength martensite heat resisting steel of thepresent invention contains 0.09-0.16% by mass of C, 0.03-0.08% of Si,0.3-0.55% of Mn, 0.2-0.7% of Ni, 10-11% of Cr, 0.3-0.55% of Mo, 2.0-2.5%of W, 0.1-0.3% of V, 0.04-0.10% of Nb, and 0.01-0.07% of N, (W/Mo) being4.0-8.0.

More preferably, the high-strength martensite heat resisting steel ofthe present invention further contains at least one of 0.015% or less ofB and 0.015% or less of Al. Also, (Mo+0.5W) is 1.3-1.7 in order tostably ensure satisfactory creep rupture strength and toughness.

In particular, with intent to increase productivity and toughness,neither Co nor B is added. Even when those elements are added, it ispreferable that the Co content is held 2.0% at maximum and the B contentis held 0.015% at maximum. Further, the Al content is preferably held0.005% or less to increase the long-time creep rupture strength.

Further, the present invention resides in a method of producing thehigh-strength martensite heat resisting steel having the above-describedsteel composition and a method of producing a rotor shaft of any of ahigh-, an intermediate- and a high- and intermediate-pressure integralsteam turbine using that steel, wherein the method includes a series ofsteps of hot plastic working, quenching, primary tempering at desiredtemperature, and secondary tempering at higher temperature than that inthe primary tempering.

The reasons why respective elements of the high-strength heat resistingcast steel according to the present invention, which can be used for therotor shafts of the high-, the intermediate-, and the high- andintermediate-pressure integral steam turbine, are limited to the aboveranges will be described below.

C is an element necessary for ensuring hardenability. In the temperingprocess, C binds with Cr, W, Mo, etc. to form M₂₃C₆- and M₆C-typecarbides at the crystal grain boundary, and also binds with Nb, V, etc.to form MX-type carbo-nitrides within grains. To obtain those effects, Cis required to be 0.05% at minimum. However, excessive addition of Ccauses excessive precipitation of the M₂₃C₆-type carbides and reducesstrength of the matrix (base material), thus decreasing high-temperaturestrength. For that reason, an upper limit of the C content is set to0.2%. In particular, a preferable range is 0.07-0.15% and a morepreferable range is 0.09-0.16%.

Si is an element that effectively acts as a deoxidizer for molten steel.However, Si promotes precipitation of the Laves phase and reducesductility due to segregation at the grain boundary, etc. For thatreason, the Si content is limited to 0.10% or less. A preferable rangeis 0.03-0.08%.

Mn is an element that effectively serves as a deoxidizer and adesulfurizer. Also, Mn improves hardenability. Further, Mn suppressesprecipitation of δ-ferrite while promoting precipitation of M₂₃C₆-typecarbides. Therefore, Mn is required to be added 0.15% at minimum.However, excessive addition of Mn deteriorates oxidation resistance. Forthat reason, an upper limit of the Mn content is set to 0.7%. Apreferable range is 0.3-0.55%.

Ni is an element that suppresses precipitation of δ-ferrite, thusproviding toughness. However, excessive addition of Ni reduces the creeprupture strength. For that reason, the Ni content is limited to0.15-1.0%. A preferable range is 0.2-0.7%.

Cr is an element that is effective in providing oxidation resistance andin precipitating M₂₃C₆-type carbides, to thereby increase thehigh-temperature strength. In order to obtain those effects, Cr isrequired to be 9% at minimum. However, excessive addition of Cr causesprecipitation of δ-ferrite and reduces fatigue strength. For thatreason, the Cr content is limited to 9.5-12.0%. A preferable range is10-11%.

Mo improves hardenability and increases temper softening resistance.Also, Mo is also effective in increasing the high-temperature strengthbased on the action of promoting fine precipitation of M₂₃C₆-typecarbides and preventing aggregation. Therefore, Mo is required to be0.2% or more. In relation to the W content, however, the Mo contentshould be held 0.65% or less. A preferable range is 0.3-0.55%.

W has a more powerful action of suppressing aggregation of M₂₃C₆-typecarbides into coarser grains in comparison with the Mo. Also, Wstrengthens the matrix with solid solution. In particular, W iseffective in increasing the high-temperature strength by adding 2.0% ormore when Co is not contained, and by adding 1.8% or more when Co iscontained. In relation to the Mo content, however, the W content shouldbe held 3.0% or less. A preferable range is 2.0-2.5%.

V is effective in precipitating a carbo-nitride of V, to therebyincrease the high-temperature strength. However, if the V contentexceeds 0.3%, carbon is excessively fixated and the amount ofprecipitated M₂₃C₆-type carbides is reduced, thus decreasing thehigh-temperature strength. For that reason, the V content is limited to0.10-0.30%. A preferable range is 0.13-0.25%.

Co contributes to not only strengthening the matrix with solid solution,but also suppressing precipitation of δ-ferrite. Addition of 0.1% ormore of Co noticeably increases the high-temperature strength, and oneof the reasons why such effect is developed is presumably attributableto the interaction with W. In other words, that effect is a specificphenomenon occurred when W is contained 1.8% or more. In creep causingenvironment at high temperature and long time, however, excessiveaddition of Co causes aggregation of M₂₃C₆-type carbides into coarsergrains at the crystal grain boundary, thus reducing the creep rupturestrength and also reducing the ductility. This results in deterioratingproductivity and increasing the cost. For that reason, an upper limit ofthe Co content is set to 2.0%. A preferable range is 0.5-1.9%.

Nb forms NbC and contributes to generating finer crystal grains. Also, apart of Nb is brought into the solid solution state in the quenchingstep and forms NbC in the tempering step, thus increasing thehigh-temperature strength. Therefore, Nb is required to be added 0.03%or more. As with V, however, if the Nb content exceeds 0.15%, carbon isexcessively fixated and the amount of precipitated M₂₃C₆-type carbidesis reduced, thus decreasing the high-temperature strength. For thatreason, the Nb content is limited to 0.03-0.15%. A preferable range is0.04-0.10%.

N has the actions of precipitating a nitride of V and increasing thehigh-temperature strength in the solid solution state based on the ISeffect (interaction between an interstitial solid solution element and asubstitutive solid solution element) in cooperation with Mo and W.Therefore, N is required to be added 0.02% at minimum. However, additionof N in excess of 0.1% reduces ductility. For that reason, the N contentis limited to 0.02-0.1%. A preferable range is 0.04-0.07%.

B has the action of coming into the solid solution state in M₂₃C₆,thereby suppressing aggregation of M₂₃C₆-type carbides into coarsergrains and increasing the high-temperature strength throughstrengthening of the grain boundary. However, addition of B in excess of0.015% impairs weldability. Further, addition of B deterioratesproductivity and increases the cost. For that reason, an upper limit ofthe B content is set to 0.015%. Further, in the heat resisting steel ofthe present invention having superior creep rupture strength in thetemperature range of 600-630° C., high toughness is obtaining by addingneither Co nor B. Thus, the electroslag remelting is not required andthe production cost can be reduced. A preferable range is 0.008-0.015%.

Al is added as a deoxidizer and a crystal grain reducing agent(refiner). However, Al is a nitride-forming element and fixates N, whicheffectively acts to increase the creep rupture strength, therebyreducing the long-time creep rupture strength in a high temperaturerange. Also, Al promotes precipitation of the Laves phase in the form ofa brittle intermetallic compound made of mainly W, and causesprecipitation of the Laves phase at the crystal grain boundary, thusreducing the long-time creep rupture strength. In particular, when finercrystal grains are formed, the Laves phase is continuously precipitatedalong the crystal grain boundary. Accordingly, an upper limit of the Alcontent is set to 0.015%. A preferable range is 0.010% or less and amore preferable range is 0.0005-0.005%.

Mo and W have the similar effect in point of increasing thehigh-temperature strength and are added in a combined manner. Withimportance focused on the creep rupture strength in the high temperaturerange, however, the W content is relatively increased. In the combinedaddition of Mo and W, they are added such that (Mo+0.5W) is preferably1.3-1.7 and more preferably 1.5-0.1 within the above-mentionedrespective composition ranges of Mo and W. Here, (Mo+0.5W) is defined asthe Mo equivalent. Taking into account the correlation between W and Mo,it is possible to ensure the creep rupture strength and obtain thetoughness by setting a (W/Mo) ratio to be 4.0-10.0 within theabove-mentioned range of the Mo equivalent. Although those bothcharacteristics are increased and decreased depending the addedelements, satisfactory characteristics can be obtained when the (W/Mo)ratio is 4.0-8.0 in the same composition system.

The Cr equivalent expressed by the following formula is preferably4-10.5 and more preferably 6.5-9.5:

Cr equivalent=−40 C %−30 N %−2 Mn %−4 Ni %−2 Co %+Cr %+6 Si %+4 Mo %+1.5W %+11 V %+5 Nb %+2 Ta %

Further, the present invention resides in a rotor shaft for use in ahigh-pressure steam turbine, an intermediate-pressure steam turbine, anda high- and intermediate-pressure integral steam turbine, wherein therotor shaft is produced by preparing an ingot by vacuum melting, vacuumcarbon deoxidation melting, or as required, electroslag remelting, andby performing successive steps of hot forging at 850-1150° C., heatingat 900-1150° C., preferably 1000-1100° C., after rough cutting of theingot surface, quenching at a cooling rate of 50-150° C./hour at acentral hole by water spraying, primary tempering at 500-620° C.,preferably 550-650° C., followed by subsequent furnace cooling, andsecondary tempering at temperature of 630-750° C., preferably 660-740°C., higher than that in the primary tempering, followed by subsequentfurnace cooling.

In the steam turbine rotor shaft made of the 12 mass %-Cr martensitesteel according to the present invention, a buildup weld layer ispreferably formed on the surface of a matrix (base material) in ajournal portion of the rotor shaft by using a welding material made ofCr—Mo low-alloy steel that has a high bearing characteristic. It ispreferable that the buildup weld layer is formed in 3-10 multi-layers.The Cr content of the welding material is gradually reduced from thefirst layer to any of the second to fourth layers. The fourth andsubsequent layers are welded by using the welding material made of thesteel having the same Cr content. Further, the Cr content of the weldingmaterial used for welding the first layer is reduced about 2-6% by massfrom that of the base material, and the Cr content in the fourth andsubsequent weld layers is set to 0.5-3% by mass (preferably 1-2.5% bymass).

The buildup welding is preferable to improve the bearing characteristicof the journal portion because of having the highest safety, but thejournal portion may have a shrink-fitting or press-fitting structure ofa sleeve made of low-alloy steel containing 1-3% of Cr. From theviewpoint of forming many weld layers and gradually reducing the Crcontent in the weld layers, the number of the weld layers is preferablythree or more. However, even if ten or more weld layers are formed, theeffect cannot be obtained in excess of a saturated level. In order toform the weld layers having a required thickness, at least five buildupweld layers are preferably formed except for an allowance for finalfinish by cutting. In addition, preferably, the third and subsequentweld layers are made of the steel mainly having the tempered martensitestructure, and the fourth and subsequent weld layers are made of thesteel containing 0.03-0.1% by mass of C, 0.3-1% of Si, and 0.3-1.5% ofMn.

Still further, the present invention resides in a first-stage rotorblade and a first-stage stator nozzle of the high-pressure steamturbine, the intermediate-pressure steam turbine, and the high- andintermediate-pressure integral steam turbine, wherein the first-stagerotor blade and the first-stage stator nozzle are each produced bypreparing an ingot by any of vacuum melting, vacuum carbon deoxidationmelting, and electroslag remelting, and by performing successive stepsof hot forging at 850-1150° C., heating at 900-1150° C., quenching at acooling rate of 300-600° C./hour by oil cooling, primary tempering at500-620° C., and secondary tempering at temperature of 630-750° C.higher than that in the primary tempering.

Still further, the present invention resides in a high-pressure steamturbine comprising a rotor shaft, rotor blades implanted to the rotorshaft, stator nozzles for guiding inflow of steam toward the rotorblades, and an inner casing for holding the stator nozzles, wherein therotor blades are disposed in eight or more stages on one side with thefirst stage being of the double-flow type, and the rotor shaft alone orthe rotor shaft and at least a first-stage rotor blade and stator nozzleof the rotor blades and the stator nozzles are made of theabove-described martensite heat resisting steel.

Still further, the present invention resides in an intermediate-pressuresteam turbine comprising a rotor shaft, rotor blades implanted to therotor shaft, stator nozzles for guiding inflow of steam toward the rotorblades, and an inner casing for holding the stator nozzles, wherein therotor blades are disposed in five or more stages on each of left andright sides in bilaterally symmetrical arrangement and have adouble-flow structure with the first stage implanted to a centralportion of the rotor shaft, and the rotor shaft alone or the rotor shaftand at least a first-stage rotor blade and stator nozzle of the rotorblades and the stator nozzles are made of the above-described martensiteheat resisting steel.

Still further, the present invention resides in a high- andintermediate-pressure integral steam turbine comprising a rotor shaft,rotor blades implanted to the rotor shaft, stator nozzles for guidinginflow of steam toward the rotor blades, and an inner casing for holdingthe stator nozzles, wherein the rotor blades are disposed in seven ormore stages on the high-pressure side and five or more stages on theintermediate-pressure side, and the rotor shaft alone or the rotor shaftand at least a first-stage rotor blade and stator nozzle of the rotorblades and the stator nozzles are made of the above-described martensiteheat resisting steel.

Still further, the present invention resides in a steam turbine powerplant including any of a set of a high-pressure steam turbine, anintermediate-pressure steam turbine, and two low-pressure steam turbinesconnected in tandem, and a set of a high- and intermediate-pressureintegral steam turbine and a low-pressure steam turbine, wherein a rotorshaft alone or a rotor shaft and at least a first-stage rotor blade andstator nozzle of the rotor blades and the stator nozzles are made of theabove-described martensite heat resisting steel.

In the steam turbine power plant according to the present invention,preferably, the low-pressure steam turbine includes the rotor bladesdisposed in eight or more stages on each of the left and right sides inbilaterally symmetrical arrangement and has the double-flow structurewith the first stage implanted to the central portion of the rotorshaft. The last-stage one of the rotor blades is made of martensitesteel containing 0.1-0.4% by mass of C, 0.25% or less of Si, 0.90% orless of Mn, 8.0-13.0% of Cr, 2-3% or less of Ni, 1.5-3.0% of Mo,0.05-0.35% of V, 0.02-0.20% of one or more of Nb and Ta in total, and0.02-0.10% of N, and it has tensile strength at room temperature of 120kgf/mm² or more, preferably 128.5 kgf/mm² or more. Further, the bladeheight is 36 inches or more, and a value of [blade height (inch)×numberof revolutions (rpm)] is 125,000 or more.

Such a long blade of the steam turbine must have high tensile strengthand high cycle fatigue strength to be endurable against largecentrifugal stress and vibration stress which are caused by high-speedrotation. To that end, the metal structure of a blade material is formedas the fully tempered martensite structure because the fatigue strengthis reduced if the harmful 8 ferrite structure is present. The materialcomposition is adjusted so as to hold the Cr equivalent at 10 or lessand to ensure that the metal structure of the blade material does notessentially contain the δ ferrite phase.

Further, in order to obtain the homogeneous and high-strength long bladeof the steam turbine, thermal refining is conducted by performing, aftersmelting and forging steps, quenching (preferably oil cooling) throughsteps of heating to 1000-1100° C., i.e., temperature sufficient forcomplete transform to the austenite structure, holding for preferably0.5-3 hours and subsequent quick cooling to room temperature, and two ormore stages of heat treatment, e.g., primary tempering through steps ofheating to 550-570° C., holding for preferably 1-6 hours and subsequentcooling to room temperature, and secondary tempering through steps ofheating to 560-680° C., holding for preferably 1-6 hours and subsequentcooling to room temperature.

In an inner casing of the steam turbine which is made of thehigh-strength heat resisting steel according to the present invention,preferably, the inner casing is made of high-strength martensite steelcontaining 0.06-0.16% by mass of C, 0.5% or less of Si, 1% or less ofMn, 0.2-1.0% of Ni, 8-12% of Cr, 0.05-0.35% of V, 0.01-0.15% of Nb, 2%or less of Co, 0.01-0.1% of N, 1.5% or less of Mo, 1-4% of W, and0.0005-0.003% of B. Further, the element composition is adjusted to holdthe Cr equivalent in the range of 4-10 such that 95% or more of thetempered martensite structure (i.e., 5% or less of the 8 ferrite) isobtained.

Thus, according to the present invention, it is possible to provide thehigh-strength martensite heat resisting steel which has the long-timecreep rupture strength required for the steam temperature condition of600-630° C. and toughness at room temperature, and which is suitable foruse as the material of the steam turbine rotor shaft and as large-sizedforged steel with an improvement of hot forgeability, and to the methodof producing the steel. Also, the present invention can provide thesteam turbine rotor shaft and the method of producing it, the steamturbine rotor blade and the method of producing it, the steam turbinestator nozzle and the method of producing it, as well as the steamturbine and the steam turbine power plant, including the method ofproducing the steam turbine, in which the turbine blade in a stage usingsteam to cool the rotor has a larger height by increasing thehigh-temperature tensile strength, and higher thermal efficiency isensured.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relationship between creep rupturestrength at 620° C. and 10⁵ hours and (W/Mo);

FIG. 2 is a graph showing the relationship between impact absorptionenergy at 25° C. and (W/Mo);

FIG. 3 is a graph showing the relationship between creep rupturestrength and impact absorption energy;

FIG. 4 is a cross-sectional view of a high-pressure steam turbineaccording to the present invention;

FIG. 5 is a cross-sectional view of an intermediate-pressure steamturbine according to the present invention; and

FIG. 6 is a cross-sectional view of a high- and intermediate-pressureintegral steam turbine according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The best mode for carrying out the present invention will be describedin detail below in connection with exemplary embodiments. It is to benoted that the present invention is not limited to those embodiments.

First Embodiment

Table 1, given below, shows chemical composition (% by mass) of thesteel of the present invention and comparative steels which are used inthis embodiment for comparative studies. In Table 1, samples No. 1-10represent the steel of the present invention, samples No. 11-13represent the comparative steels, and samples No. 14-19 represent theknown steels (corresponding to Patent Documents 1 and 3). As seen fromTable 1, in the samples No. 1-10 representing the steel of the presentinvention, the (W/Mo) ratio is 4.0-10.0.

Each of the steel samples shown in Table 1 was prepared by producing aningot of 50 kg in a vacuum high-frequency induction melting furnace, andforming a plate of 30 mm (t)×90 mm (w)×L by hot forging. The hot forgingwas performed under heating conditions of temperature 1150° C.×3 hoursand at the forging temperature of 1150-950° C. while the heating wasrepeated six times.

Simulating a central portion of a large-sized steam turbine rotor shaft,heat treatment was conducted by successively performing quenching of1050° C.×5 hours at a cooling rate of 100° C./hour, primary temperingthrough heating and holding of 570° C.×20 hours, and secondary temperingthrough heating and holding of 680° C.×20 hours. For each of the steelsamples thus prepared, creep rupture tests were made at varioustemperatures and the creep rupture strength at 620° C. and 10⁵ hours wascalculated from the test results by extrapolation. Also, V-notch Charpyimpact tests at room temperature (20° C.) were made on each steel sampleto obtain absorption energy.

TABLE 1 Chemical Composition (% by mass) Sample C Si Mn Ni Cr Mo W V CoNb B N Al Cr Equivalent Mo Equivalent W/Mo 1 0.12 0.06 0.50 0.25 10.160.45 2.05 0.16 1.85 0.06 0.011 0.021 0.001 6.3 1.48 4.6 2 0.13 0.06 0.500.24 10.12 0.46 2.07 0.16 1.00 0.06 0.011 0.020 0.001 7.7 1.50 4.5 30.12 0.06 0.52 0.27 10.16 0.44 2.08 0.16 1.00 0.06 0.053 0.001 7.0 1.484.7 4 0.12 0.07 0.51 0.24 10.13 0.45 2.06 0.16 0.54 0.06 0.051 0.001 8.11.48 4.6 5 0.13 0.06 0.53 0.25 10.22 0.47 2.04 0.16 0.06 0.048 0.001 8.91.49 4.3 6 0.13 0.06 0.51 0.50 10.23 0.46 2.06 0.17 0.06 0.049 0.001 8.01.49 4.5 7 0.12 0.06 0.51 0.51 10.27 0.43 2.20 0.17 1.82 0.05 0.0510.002 4.7 1.53 5.1 8 0.12 0.06 0.52 0.50 10.41 0.35 2.31 0.17 1.83 0.050.048 0.002 4.8 1.51 6.6 9 0.12 0.06 0.50 0.49 10.41 0.25 2.50 0.17 1.810.05 0.012 0.020 0.002 5.7 1.50 10.0 10 0.12 0.07 0.52 0.50 10.43 0.312.39 0.17 1.81 0.05 0.049 0.002 4.9 1.51 7.7 11 0.12 0.06 0.49 0.5010.17 0.80 1.41 0.17 0.06 0.048 0.001 8.8 1.51 1.8 12 0.12 0.07 0.510.24 10.40 0.22 2.56 0.17 0.05 0.050 0.002 9.4 1.50 11.6 13 0.12 0.070.06 0.20 10.00 0.60 1.80 0.20 0.06 0.012 0.022 0.010 11.6 1.50 3.0 140.12 0.06 0.04 0.05 10.20 0.70 1.70 0.20 3.30 0.06 0.002 0.022 0.010 6.11.55 2.4 15 0.12 0.05 0.50 0.50 10.38 0.41 1.82 0.17 0.05 0.050 0.0047.9 1.32 4.4 16 0.11 0.06 0.46 0.25 10.20 0.20 2.60 0.20 2.54 0.07 0.0160.002 5.9 1.50 13.0 17 0.11 0.06 0.46 0.25 10.20 0.20 2.60 0.20 2.520.07 0.013 0.017 0.002 5.9 1.50 13.0 18 0.14 0.28 0.52 0.60 11.09 1.240.42 0.19 0.09 0.035 0.004 10.8 1.45 0.3 19 0.12 0.08 0.06 0.20 10.000.60 1.80 0.20 3.02 0.06 0.012 0.021 0.010 5.7 1.50 3.0

Table 2, given below, shows the creep rupture strength at 620° C. and10⁵ hours and the absorption energy obtained from the results of theCharpy impact tests at 25° C. As seen from Table 2, in the samples No.1-10 representing the steel of the present invention, the creep rupturestrength at 620° C. and 10⁵ hours is relatively high in the range of10.35-13.00 kgf/mm² as a whole. The samples No. 11-13 representing thecomparative steels and the samples No. 14-19 representing the knownsteels have the creep rupture strengths varying in the range of7.30-13.26 kgf/mm². In any type of the steels, steel characteristics areclarified by dividing them into systems containing Co and B, notcontaining Co and B, and not containing Co or B. It is apparent in eachof those systems that the steel of the present invention having the(W/Mo) ratio of 4.0-10.0 has higher strength.

TABLE 2 Creep Rupture Strength Absorption Energy (20° C.) kgf/mm² J 111.72 55 2 11.79 58 3 11.26 130 4 10.46 155 5 11.03 119 6 11.85 139 711.40 137 8 12.00 132 9 13.00 42 10 11.70 128 11 10.17 145 12 12.70 11013 10.71 60 14 12.03 58 15 11.62 135 16 11.22 85 17 13.26 10 18 7.30 4519 11.34 60

Also, as seen from Table 2, in the samples No. 1-10 representing thesteel of the present invention, the absorption energy obtained from theresults of the Charpy impact tests at room temperature (20° C.) isrelatively high in the range of 55-139 J as a whole. The samples No.11-13 representing the comparative steels and the samples No. 14-19representing the known steels have the absorption energy varying in therange of 10-145 J. In any type of the steels, steel characteristics areclarified by dividing them into systems containing Co and B, notcontaining Co and B, and not containing Co or B. It is apparent in eachof those systems that the steel of the present invention having the(W/Mo) ratio of 4.0-10.0 has higher absorption energy.

FIG. 1 is a graph showing the relationship between the (W/Mo) ratio andthe creep rupture strength at 620° C. and 10⁵ hours. As seen from FIG.1, in any type of the steels, the creep rupture strength at 620° C. and10⁵ hours is noticeably increased by increasing the (W/Mo) ratio in thesteel and has a value higher than 10 kgf/mm². Thus, any type of thesteels can be satisfactorily used as the material of the rotor shaft ofthe steam turbine operated at steam temperature of 600° C. or above fromthe viewpoint of the creep rupture strength.

Also, as seen from FIG. 1, in any type of the steels, the steel systemscontaining Co and B have higher strength than the steel systems notcontaining Co and B and not containing Co or B. Further, it is apparentthat, in each of those steel systems, higher strength is obtained withan increase of the (W/Mo) ratio including the range of 4.0-10.0. In thesteel system containing Co, however, the strength is reduced when the(W/Mo) ratio exceeds 10. Further, higher creep rupture strength isobtained as the Co content increases.

FIG. 2 is a graph showing the relationship between the (W/Mo) ratio andthe absorption energy obtained from the results of the Charpy impacttests at room temperature (20° C.) As seen from FIG. 2, comparing allthe types of the steels, the energy absorption is higher in the steel ofthe present invention in which the (W/Mo) ratio is 4.0-10.0, regardlessof the steel systems containing Co and B, not containing Co and B, andnot containing Co or B. Further, the absorption energy is at minimum inthe steel system containing Co and B, and is abruptly reduced at the(W/Mo) ratio of 10 or above. Particularly, in the steel systemcontaining Co, the absorption energy is abruptly reduced when the (W/Mo)ratio exceeds 10. Thus, it is apparent that, in each of the steelsystems, the steel of the present invention having the (W/Mo) of4.0-10.0 has higher absorption energy.

FIG. 3 is a graph showing the relationship between the creep rupturestrength at 620° C. and 10⁵ hours and the absorption energy obtainedfrom the results of the Charpy impact tests at 20° C. The relationshipbetween the creep rupture strength and the absorption energy differs foreach of the steel system not containing Co and B (indicated by marks ▴and Δ), the steel system containing Co or B (indicated by marks ▪ and□), and the steel system containing Co and B (indicated by marks ▪ and◯). As seen from FIG. 3, the higher the creep rupture strength, thelower is the absorption energy. Comparing the absorption energy at thesame creep rupture strength, the absorption energy is reduced in theorder of the steel system not containing Co and B, the steel systemcontaining Co, the steel system containing B, and the steel systemcontaining Co and B.

In other words, the comparative steels and the known steels arerepresented by the samples No. 11 and 15 (containing 0.5% of Ni) whichbelong to the steel system not containing Co and B (indicated by mark▴), the sample No. 12 (containing 0.24% of Ni) which belongs to thesteel system not containing Co and B (indicated by mark ▴), the samplesNo. 16 and 14 which belong to the steel system containing Co (indicatedby mark ▪), the sample No. 13 which belongs to the steel systemcontaining B (indicated by mark ▪), and the samples No. 19 and 17 whichbelong to the steel system containing Co and B (indicated by mark ).From comparison at the same creep rupture strength, it is apparent thatthe samples No. 1-10 representing the steel of the present invention((indicated by marks Δ, □ and ◯) have higher absorption energy than thecomparative steels and the known steels belonging to the respective samesteel systems. Accordingly, the steel of the present invention hashigher absorption energy than levels given by characteristic linesrepresenting the comparative steels and the known steels. Comparing fromanother aspect, the steel of the present invention has higher creeprupture strength than the comparative steels and the known steels at thesame absorption energy.

Thus, the steel of the present invention has the long-time creep rupturestrength required for the steam temperature condition of 600-630° C. andtoughness at room temperature, and it is suitable for use as thematerial of the steam turbine rotor shaft and as the large-sized forgedsteel with an improvement of hot forgeability.

Second Embodiment

FIG. 4 is a cross-sectional view of a high-pressure steam turbine (HP)using the high-strength martensite heat resisting steel according to thepresent invention as a rotor shaft material. FIG. 5 is a cross-sectionalview of an intermediate-pressure steam turbine (IP) using thehigh-strength martensite heat resisting steel according to the presentinvention as a rotor shaft material. In this second embodiment, the HPand the IP are connected in tandem to constitute a steam turbine powerplant having steam temperature of 625° C. and output capacity of 1050MW. A low-pressure steam turbine is of the cross-compound four-flowexhaust type, and the blade height in the last stage thereof is 43inches. More specifically, the steam turbine power plant can beconstituted by a set of (HP)-(IP)-generator and a set of twolow-pressure steam turbines (LP)-generator, each set operating at therotation speed of 3000 rpm, or by a set of (HP)-(LP)-generator and a setof (IP)-(LP)-generator, each set operating at the rotation speed of 3000rpm. The steam temperature and pressure in the HP are 625° C. and 250kgf/cm². In the IP, the steam temperature is heated to 625° C. by areheater and operation is performed at pressure of 45-65 kgf/cm². Steamhaving temperature of 400° C. enters the LP and is sent to a condenserunder vacuum of 722 mmHg at 100° C. or below.

The high-temperature and high-pressure steam turbine power plantaccording to this embodiment comprises mainly a coal firing boiler, oneHP, one IP, two LPs, a condenser, a condensing pump, a low-pressurefeedwater heater system, a deaerator, a booster pump, a feedwater pump,and a high-pressure feedwater heater system. More specifically, ultrahigh-temperature and high-pressure steam generated in the boiler entersthe HP in which motive power is produced. Then, the steam is reheated bythe boiler and enters the IP in which motive power is produced. Thesteam exhausted from the IP enters the LP in which motive power isproduced, followed by being condensed in the condenser. The condensedwater is sent to the low-pressure feedwater heater system and thedeaerator by the condensing pump. The feedwater deaerated in thedeaerator is sent to the high-pressure feedwater heater system by thebooster pump and the condensing pump. After the water temperature israised in the high-pressure feedwater heater system, the feedwater isreturned to the boiler. In the boiler, the feedwater is converted tohigh-temperature and high-pressure steam through an economizer, anevaporator and a superheater.

The HP includes a high-pressure inner compartment (casing) 18, ahigh-pressure outer compartment (casing) 19 surrounding the innercompartment 18, and a high-pressure rotor shaft 20 provided withhigh-pressure rotor blades 16 implanted to it and disposed inside thosecasings. High-temperature and high-pressure steam obtained by a boilerpasses through a main steam pipe and flows into a main steam inlet 28through a flange and elbow 25 constituting a main steam section. Thesteam is then introduced to the rotor blade in the double-flow firststage through a nozzle box 38. The first stage has a double-flowstructure, and the rotor blades are disposed in eight stages on oneside. Stator nozzles are disposed corresponding to the rotor blades.

The IP is used to rotate the generator in cooperation with the HP byutilizing steam obtained by heating the steam, which is exhausted fromthe HP, to 625° C. again by a reheater. Similarly to the HP, the IP hasan intermediate-pressure inner compartment (casing) 21 and anintermediate-pressure outer compartment (casing) 22, and furtherincludes stator nozzle corresponding to the intermediate-pressure rotorblades 17. The intermediate-pressure rotor blades 17 are disposed sixstages in each of the left and right sides in bilaterally symmetricarrangement with a double-flow structure in which the first-stage rotorblade is implanted to a central portion of the intermediate-pressurerotor shaft 24.

According to this embodiment, in each of the HP and IP, the rotor shaft,the first-stage rotor blade, and the first-stage stator nozzle are madeof one sample steel of the present invention in Table 1 described above,i.e., the 12%-Cr steel containing Co and B. The rotor shafts of the HPand the IP have similar characteristics to those in the above-describedfirst embodiment. The first-stage rotor blade and the first-stage statornozzle are produced through the steps of quenching by oil cooling afterheating to a temperature level similar to that in the case of the rotorshaft, and tempering at 650-750° C. Thus, the first-stage rotor bladeand the first-stage stator nozzle have slightly higher creep rupturestrength and impact values than those of the rotor shaft.

The rotor shafts of the high-pressure steam turbine and theintermediate-pressure steam turbine were produced as follows. First, 30tons of the heat resisting cast steel shown in Table 1 was smelted in anelectric furnace, and after carbon vacuum deoxidation, the smelted steelwas cast into a mold, followed by forming an electrode rod withelongation forging. Then, electroslag remelting was performed to smeltthe cast steel from an upper portion toward a lower portion by using theelectrode rod, followed by elongation forging into the rotor shape. Theelongation forging was performed at temperature of 1150° C. or below inorder to prevent forging cracks.

After annealing the forged steel, the steel was subjected to steps ofheating and holding to 1050° C. while slowly rotating it at speed of 1(rotation/minute), quenching (cooling rate of 100° C./hour at a centralportion) by water spraying while slowly rotating it at a similar speed,primary tempering at 570° C., and secondary tempering at 690° C. Therotor shafts were then obtained by cutting into the respective shapesshown in FIGS. 4 and 5. In this embodiment, each rotor shaft was formedsuch that the upper side of the electroslag steel ingot becomes thefirst-stage blade side and the lower side becomes the last-stage bladeside.

As a result of examining the central portion of each rotor shaftobtained according to this embodiment, it was proved that the rotorshaft sufficiently satisfied the characteristics (i.e., the creeprupture strength at 620° C. and 10⁵ hours≧10 kgf/mm² and the impactabsorption energy at 20° C.≧1.5 kgf-m) required for the high- andintermediate-pressure turbine rotors. Thus, it was proved that the steamturbine rotor capable of being used in steam at 600-630° C. could beproduced.

Two LPs are connected in tandem and have substantially the samestructure. In each LP, last-stage and other rotor blades are disposed ineight stages in each of the left and right sides and are arranged insubstantially bilateral symmetry. Stator nozzles are disposedcorresponding to the rotor blades. The last-stage rotor blade has anairfoil height of 43 inches and is produced through a series of steps ofsmelting by the electroslag remelting process, forging, and heattreatment. Such a long blade is made of martensite steel containing0.08-0.18% by mass of C, 0.25% or less of Si, 0.90% or less of Mn,8.0-13.0% of Cr, 2-3% of Ni, 1.5-3.0% of Mo, 0.05-0.35% of V, 0.02-0.20%of one or more of Nb and Ta in total, and 0.02-0.10% of N. Also, thelong blade exhibits the tensile strength at room temperature of 120kgf/mm² or more and has the fully tempered martensite structure. Morepreferably, the tensile strength is 128.5 kgf/mm² or more and theV-notch Charpy impact value at 20° C. is 4 kgf-m/cm² or more. An airfoilof the long blade having the airfoil height of 43 inches, against whichhigh-speed steam impinges, is coated with an erosion shield formed byjoining a stellite sheet made of a Co-base alloy by welding in order toprevent erosion caused by water droplets in the steam.

A low-pressure rotor shaft is made of forged steel having the fullytempered bainite structure of a super-clean material containing 3.75% ofNi, 1.75% of Cr, 0.4% of Mo, 0.15% of V, 0.25% of C, 0.05% of Si, and0.10% of Mn, the balance being Fe. Each of the rotor blades and thestator nozzles in other stages than the last stage is made of the 12%-Crsteel containing 0.1% of Mo. Inner and outer casings are each made of0.25%-C cast steel.

Further, in this embodiment, a shaft of a generator with output capacityof 1050-MW class is made of higher-strength steel having the fullytempered bainite structure and containing 0.15-0.30% of C, 0.1-0.3% ofSi, 0.5% or less of Mn, 3.25-4.5% of Ni, 2.05-3.0% of Cr, 0.25-0.60% ofMo, and 0.05-0.20% of V. Further, the tensile strength at roomtemperature is 93 kgf/mm² or more, preferably 100 kgf/mm² or more, and50%-FATT (Fracture Appearance Transition Temperature) is 0° C. or below,preferably −20° C. or below.

A central hole is formed in the rotor shaft of each of the HP, IP and LPso that the presence or absence of defects can be checked through thecentral hole by ultrasonic inspection, visual inspection and/orfluorescence flaw detection. Alternatively, the central hole may beomitted because defects can also be detected by ultrasonic inspectionfrom an outer surface of the rotor shaft.

Moreover, in this embodiment, the Cr—Mo low-alloy steel wasbuildup-welded on a journal portion of each rotor shaft of the HP and IPto improve bearing characteristics. A coated arc-welding electrode wasused as a welding electrode for the buildup welding.

The buildup welding was performed to form eight layers. The thickness ofeach layer was 3-4 mm and the total thickness was about 28 mm with aweld surface cut about 5 mm by grinding. Welding conditions were setsuch that each of the preheating temperature, the inter-passtemperature, and the start temperature of stress removing annealing (SR)was 250-350° C. and the SR was performed under conditions of heating andholding of 630° C.×36 hours. First to third layers were formed by usingcoated arc-welding electrodes made of 8%-Cr-0.5%-Mo steel, 5%-Cr-0.5%-Mosteel, and 2.3%-Cr-1%-Mo steel, respectively, and fourth to eighthlayers were each formed by using a coated arc-welding electrode made of1.3%-Cr-0.76%-Mo steel. Those welding materials contained 0.03-0.07% ofC, 0.4-0.8% of Si, and 0.5-1.0% of Mn.

The first-stage rotor blade and the first-stage stator nozzle in each ofthe HP and IP were also produced by smelting, in a vacuum arc meltingfurnace, the heat resisting steel of the present invention containing Coand B, shown in Table 1 described above, and forming the steel into theblade and nozzle blank shape (with a width of 150 mm, a height of 50 mm,and a length of 1000 mm) by elongation forging. Further, the forgedsteel was subjected to heating to 1050° C., oil quenching, and temperingat 690° C. Thereafter, the forged steel was cut into the predeterminedshape.

Also, it was confirmed that the first-stage rotor blade of each of theHP and IP sufficiently satisfied the required characteristics (i.e., thecreep rupture strength at 625° C. and 10⁵ hours≧15 kgf/mm²). Thus, itwas proved that the steam turbine blade capable of being used in steamat 620° C. or above could be produced.

Inner casings of high- and intermediate-pressure sections, a casing of amain steam valve, and a casing of a steam control valve were eachproduced through the steps of smelting, in an electric furnace,heat-resisting cast steel of 0.12% C-9% Cr-0.6% Mo-1.7% W—B, ladlerefining, and casting into a sand mold. By sufficiently performingrefining and deoxidation before the casting step, the cast steel couldbe obtained which contained no casting defects such as shrinkage.

Further, as a result of examining characteristics of the inner casings,the casing of the main steam valve, and the casing of the steam controlvalve, it was confirmed that each of those casings satisfied therequired characteristics (i.e., the creep rupture strength at 625° C.and 10⁵ hours≧10 kgf/mm² and the impact absorption energy at 20° C.≧1kgf-m) and could be satisfactorily welded. Thus, it was proved that thesteam turbine casing capable of being used in steam at 620° C. or abovecould be produced.

According to the second embodiment, it is possible to provide the steamturbine rotor shaft having the long-time creep rupture strength requiredfor the steam temperature condition of 600-630° C. and toughness at roomtemperature, and the method of producing the rotor shaft, the steamturbine rotor blade having the required characteristics and the methodof producing it, as well as the steam turbine stator nozzle having therequired characteristics and the method of producing it. Further, it ispossible to provide the steam turbine and the steam turbine power plant,including the method of producing the steam turbine, in which theturbine blade in the stage using steam to cool the rotor has a largerheight by increasing the high-temperature tensile strength, and higherthermal efficiency is ensured.

Third Embodiment

FIG. 6 is a cross-sectional view of a high- and intermediate-pressureintegral steam turbine. This third embodiment relates to a steam turbinepower plant with steam temperature of 620° C. and output capacity of 600MW. The power plant of this third embodiment is of the tandem compounddouble-flow type, and the last-stage blade height in the LP is 43inches. A rotation speed of 3000 rpm is obtained by the high- andintermediate-pressure integral steam turbine (HP-IP) and one LP (C) ortwo LPs (D). The steam temperature and pressure in the high-pressuresection (HP) are 600° C. and 250 kgf/cm². In the intermediate-pressuresection (IP), the steam temperature is heated to 600° C. by a reheaterand operation is performed at pressure of 45-65 kgf/cm². The steamtemperature in the low-pressure section (LP) is 400° C., and steam inthe LP is sent to a condenser under vacuum of 722 mmHg at 100° C. orbelow.

The high-pressure side steam turbine (HP) includes an inner compartment(casing) 18 and an outer compartment (casing) 19 surrounding the innercasing 18. The intermediate-pressure steam turbine (IP) includes aninner compartment (casing) 21 and an outer compartment (casing) 22surrounding the inner casing 21. A high- and intermediate-pressureintegral rotor shaft 23 provided with high-pressure rotor blades 16 andintermediate-pressure rotor blades 17 both implanted to the rotor shaftis disposed inside those casings. The high-pressure and high-temperaturesteam obtained by the boiler passes through a main steam pipe and flowsinto a main steam inlet 28 through a flange and elbow 25 constituting amain steam section. The steam is then introduced to the high-pressurerotor blade 16 in the first stage of the high-pressure side steamturbine through a nozzle box 38. The rotor blades are disposed in eightstages in the HP on the left side in FIG. 6 and six stages in the IP onthe right side in FIG. 6. Stator nozzles are disposed corresponding tothe rotor blades.

In this third embodiment, each of the rotor shaft, the first-stage rotorblade, and the first-stage stator nozzle is made of the 12%-Cr steelcontaining Co and B among the steel samples of the present inventionshown in Table 1 described above.

The rotor shaft of the high- and intermediate-pressure integral steamturbine was produced as follows. First, 30 tons of the 12%-Cr steelshown in Table 1 was smelted in an electric furnace, and after carbonvacuum deoxidation, the smelted steel was cast into a mold, followed byforming an electrode rod with elongation forging. Then, electroslagremelting was performed to smelt the cast steel from an upper portiontoward a lower portion by using the electrode rod, followed byelongation forging into the rotor shape. The elongation forging wasperformed at temperature of 1150° C. or below in order to preventforging cracks. After annealing the forged steel, the steel wassubjected to steps of quenching by water spray cooling after heating to1050° C., two stages of tempering at 570° C. and 690° C. The rotor shaftwas then obtained by cutting into the shape shown in FIG. 5. Materialsand production conditions for the other components were the same asthose in the second embodiment. Further, buildup welding was alsoperformed on a bearing journal portion in a similar manner. In addition,the rotor shaft had the same characteristics as those described above inthe first embodiment.

The first-stage rotor blade and the first-stage stator nozzle wereproduced through the steps of quenching by oil cooling after heating toa temperature level similar to that in the case of the rotor shaft, andtempering at 650-750° C. Thus, the first-stage rotor blade and thefirst-stage stator nozzle had slightly higher creep rupture strength andimpact values than those of the rotor shaft.

The IP heats the steam discharged from the HP again to 600° C. by areheater and rotates the generator in cooperation with the HP by usingthe heated steam.

One LP is connected to the HP-LP. Last-stage and other rotor blades aredisposed in six stages in each of the left and right sides and arearranged in substantially bilateral symmetry. Stator nozzles aredisposed corresponding to the rotor blades. The last-stage rotor bladehas an airfoil height of 43 inches and is made of the 12%-Cr steel thatis similar to that used in the second embodiment. Also, as in the secondembodiment, the last-stage rotor blade in this third embodiment haserosion shields made of stellite steel, which are welded at two pointsin the front and rear sides of the rotor blade by electron beam weldingor TIG welding.

Each of the rotor shaft of the low-pressure steam turbine and the rotorblades and the stator nozzles in other stages than the last stage isproduced in the same manner as that described above in the secondembodiment.

The concept of this third embodiment can be similarly applied to anothertype of power plant, e.g., a large-capacity power plant of 1000-MW classin which the steam inlet temperature of the high- andintermediate-pressure integral steam turbine is 610° C. or above and thesteam inlet temperature and the steam outlet temperature of thelow-pressure integral steam turbine are about 400° C. and about 60° C.,respectively.

While this third embodiment has been described in connection with thepower plant of the tandem compound double-flow type, it can be modifiedbased on the same concept to, e.g., a power plant with output capacityof 1050-MW class in which two low-pressure steam turbines are connectedin tandem. In that case, a generator shaft is made of higher-strengthsteel having the fully tempered bainite structure, the tensile strengthat room temperature of 93 kgf/mm² or more, preferably 100 kgf/mm² ormore, and 50%-FATT (Fracture Appearance Transition Temperature) of 0° C.or below, preferably −20° C. or below, as in the second embodiment.

According to the third embodiment, it is possible to provide the steamturbine rotor shaft having the long-time creep rupture strength requiredfor the steam temperature condition of 600-630° C. and toughness at roomtemperature, and the method of producing the rotor shaft, the steamturbine rotor blade having the required characteristics and the methodof producing it, as well as the steam turbine stator nozzle having therequired characteristics and the method of producing it. Further, it ispossible to provide the steam turbine and the steam turbine power plant,including the method of producing the steam turbine, in which theturbine blade in the stage using steam to cool the rotor has a largerheight by increasing the high-temperature tensile strength, and higherthermal efficiency is ensured.

1. A high-strength martensite heat resisting steel containing 0.07-0.20%by mass of C, 0.1% or less of Si, 0.15-0.7% of Mn, 0.15-7% of Ni,9.5-12.0% of Cr, 0.20-0.65% of Mo, 1.8-3.0% of W, 0.1-0.3% of V,0.03-0.15% of Nb, 0.01-0.10% of N, 0.5-2.0% of Co, and 0.008-0.015% ofB, wherein a ratio of W/Mo is 4.0-10.0, thereby causing thehigh-strength martensite heat resisting steel to have a minimum creeprupture strength and toughness, and wherein the balance of thehigh-strength martensite heat resisting steel is Fe and unavoidableimpurities.
 2. A high-strength martensite heat resisting steelcontaining 0.07-0.20% by mass of C, 0.1% or less of Si, 0.15-0.7% of Mn,0.15-0.7% of Ni, 9.5-12.0% of Cr, 0.20-0.65% of Mo, 1.8-3.0% of W,0.5-2.0% of Co, 0.1-0.3% of V, 0.03-0.15% of Nb, and 0.01-0.10% of N,wherein a ratio of W/Mo is 4.0-10.0, thereby causing the high-strengthmartensite heat resisting steel to have a minimum creep rupture strengthand toughness, and wherein the balance of the high-strength martensiteheat resisting steel is Fe and unavoidable impurities.
 3. Ahigh-strength martensite heat resisting steel containing 0.09-0.16% bymass of C, 0.03-0.08% of Si, 0.3-0.55% of Mn, 0.2-0.7% of Ni, 10-11% ofCr, 0.3-0.55% of Mo, 2.0-2.5% of W, 0.1-0.3% of V, 0.04-0.10% of Nb, and0.01-0.07% of N, 0.5-2.0% of Co, and 0.008-0.015% of B, wherein theratio of W/Mo is 4.0-8.07, thereby causing the high-strength martensiteheat resisting steel to have a minimum creep rupture strength andtoughness, and wherein the balance of the high-strength martensite heatresisting steel is Fe and unavoidable impurities.
 4. A high-strengthmartensite heat resisting steel containing 0.09-0.16% by mass of C,0.03-0.08% of Si, 0.3-0.55% of Mn, 0.2-0.7% of Ni, 10-11% of Cr,0.3-0.55% of Mo, 2.0-2.5% of W, 0.1-0.3% of V, 0.04-0.10% of Nb,0.01-0.07% of N, and 0.5-2.0% of Co, wherein a ratio of (W/Mo) is4.0-8.0, thereby causing the high-strength martensite heat resistingsteel to have a minimum creep rupture strength and toughness, andwherein the balance of the high-strength martensite heat resisting steelis Fe and unavoidable impurities.
 5. (canceled)
 6. A method of producinga high-strength martensite heat resisting steel containing 0.07-0.20% bymass of C, 0.1% or less of Si, 0.15-0.7% of Mn, 0.15-0.7% of Ni,9.5-12.0% of Cr, 0.20-0.65% of Mo, 1.8-3.0% of W, 0.1-0.3% of V,0.03-0.15% of Nb, 0.01-0.10% of N, 0.5-2.0% of Co, and 0.008-0.015% ofB, wherein the ratio of W/Mo is 4.0-10.0, thereby causing thehigh-strength martensite heat resisting steel to have a minimum creeprupture strength and toughness, wherein the balance of the high-strengthmartensite heat resisting steel is Fe and unavoidable impurities, andwherein the method includes a series of steps of hot plastic working,quenching, primary tempering, and secondary tempering at highertemperature than that in the primary tempering.
 7. A method of producinga high-strength martensite heat resisting steel containing 0.07-0.20% bymass of C, 0.1% or less of Si, 0.15-0.7% of Mn, 0.15-0.7% of Ni,9.5-12.0% of Cr, 0.20-0.65% of Mo, 1.8-3.0% of W, 0.5-2.0% of Co,0.1-0.3% of V, 0.03-0.15% of Nb, and 0.01-0.10% of N, wherein the ratioof W/Mo is 4.0-10.0, thereby causing the high-strength martensite heatresisting steel to have a minimum creep rupture strength and toughness,wherein the balance of the high-strength martensite heat resisting steelis Fe and unavoidable impurities, and wherein the method includes aseries of steps of hot plastic working, quenching, primary tempering,and secondary tempering at higher temperature than that in the primarytempering.
 8. The method of producing the high-strength martensite heatresisting steel containing 0.09-0.16% by mass of C, 0.03-0.08% of Si,0.3-0.55% of Mn, 0.2-0.7% of Ni, 10-11% of Cr, 0.3-0.55% of Mo, 2.0-2.5%of W, 0.1-0.3% of V, 0.04-0.10% of Nb, 0.01-0.07% of N, 0.5-2.0% of Co,and 0.008-0.015% of B, wherein a ratio of (W/Mo) is 4.0-8.0 therebycausing the high-strength martensite heat resisting steel to have aminimum creep rupture strength and toughness, wherein the balance of thehigh-strength martensite heat resisting steel is Fe and unavoidableimpurities, and wherein the method includes a series of steps of hotplastic working, quenching, primary tempering, and secondary temperingat a higher temperature than that in the primary tempering.
 9. A methodof producing the high-strength martensite heat resisting steelcontaining 0.09-0.16% by mass of C, 0.03-0.08% of Si, 0.3-0.55% of Mn,0.2-0.7% of Ni, 10-11% of Cr, 0.3-0.55% of Mo, 2.0-2.5% of W, 0.1-0.3%of V, 0.04-0.10% of Nb, 0.01-0.07% of N, and 0.5-2.0% of Co, wherein aratio of (W/Mo) is 4.0-8.0, thereby causing the high-strength martensiteheat resisting steel to have a minimum creep rupture strength andtoughness, wherein the balance of the high-strength martensite heatresisting steel is Fe and unavoidable impurities, and wherein the methodincludes a series of steps of hot plastic working, quenching, primarytempering, and secondary tempering at a higher temperature than that inthe primary tempering.
 10. (canceled)
 11. (canceled) 12-13. (canceled)14. (canceled)
 15. (canceled) 16-21. (canceled)